1. Field of the Invention
This invention relates to the art of making oxynitrides and, more particularly, to the making of silicon yttrium oxynitrides.
2. Description of the Prior Art
Silicon yttrium oxynitrides typically are made as byproducts of heating a ternary system of silicon nitride and oxide sintering aids without melting the silicon nitride (see U.S. Pat. No. 4,102,698). Alternatively, stoichiometric amounts of silicon nitride and oxides can be heated to form specific oxynitrides, again without melting the silicon nitride (see U.S. Pat. No. 4,501,723). Unfortunately, both of these methods do not yield the oxynitride in the desired purity condition (greater than 99.98%) needed for improved grain boundary control when incorporated into ceramics such as silicon nitride and are particle size dependent.
With respect to purity, the lack of it stems primarily from the inability to provide starting materials with both sufficient overall purity and sufficient upper limit on contaminant content and contaminant particle size. Silicon nitride, as a starting material, should have a purity in excess of 99.98%. Unfortunately, silicon nitride is predominantly formed by nitriding raw silicon powder along with other ingredients, and the starting silicon powder is typically commercially available in purity forms up to only 98%. The major trace metal contaminants normally experienced with such commercially available silicon powder include: iron in an amount of 1.0%, aluminum--0.5%, manganese--0.09%, and calcium--0.01%. Nonmetallic contaminants are usually present in amounts of 0.2% carbon and 2.0% oxygen. In addition and of significance is the fact that some of these impurity particles can be as large as 100 microns. These impurities of trace metals, oxygen and carbon, as well as halides or sulfur, in such early starting material find their way into the resultant silicon nitride ceramic, affect the oxynitrides formed, and eventually cause catastrophic failure of the ceramic. Moreover, contaminants can be introduced by attrition during mechanical grinding of the silicon powder or silicon nitride powder; the milling media or compacters wear during such mechanical manipulations causing undesirable compositional changes or lack of homogeniety in the grain boundary of the resulting ceramic and ultimately degradation of high temperature strength.
The purity problem has another aspect, the incorporation of impurities as a result of processing rather than the starting materials. First, the wide particle size distribution of conventional starting powders presents variable surface areas throughout the commercially available starting powder, which limits effective oxygen control so as to consistently create not only the desired silicon-yttrium-oxynitride phases, but also attendent oxide complexes; this results in strength variations in the final product.
Secondly, inadequate maximum particle size control of the starting materials leads to problems in the ceramic. In the prior art methods of manufacture of the starting materials, dry milling is employed to mix the ingredients and to provide an increased particle size distribution by comminution through the milling action. Dry milling promotes a broader particle size distribution which, in turn, provides better packing density for the powder material. However, in dry milling, the powders typically cake along the sides of the container and result in unsatisfactory maximum particle size control and homogeneity. With such a dry milled powder, attrited trace metal impurities and/or inadequate nitriding of large silicon particles will lead to the formation of silicides during processing which become sites for grain growth or critical flaws in the final ceramic. To show the lack of maximum particle size control (over 20 microns), representative starting material particle sizes are given. Silicon nitride powder used to make the prior art oxynitrides has an average starting particle size usually in the range of 7-10 microns which permits particles up to 150 microns to be present along with finer particles. [Average or mean particle size means 50% of the particles will be above the mean value and 50% will be below; this specifies an average size point, but leaves undefined the particle size above the average point.] Yttria, which is added to the silicon nitride to make the prior art oxynitrides, usually is available on the commercial market with an average particle size of about 0.04 microns and can have particles present as large as 40 microns. The SiO.sub.2, which may be added to the mixture, usually has a submicron average particle size, but allows maximum particles to be present as large as 50 microns. The presence of large particles inhibits proper gas phase formation of the oxynitride.
These powder mixtures containing a major component and several minor components with broad range and different particle sizes are generally recognized as difficult to disperse by conventional blending and milling techniques. The overall impurity levels, the maximum contaminant particle size, and the homogeneity of the powder mixture are important in determining the ceramic's performance and reliability. Prior art wet milling techniques are disadvantageous because the resulting powder typically becomes contaminated with organic residues.
What is needed is a method by which oxynitrides may be selectively and consistently made in an ultra-high purity condition and in a much finer and uniform particle size than has been possible heretofore.
It is an object of this invention to make ultrapure oxynitrides of specific phase and in a much finer and uniform particle size than has been possible heretofore.